With the development of high-speed information technologies, integrated optical devices are gradually replacing traditional micro-optical devices, and become core structural units in fields such as optical communications, optical computing, and optical sensing in future. Using a silicon waveguide as an example, due to a high difference between refractive indexes of core layer silicon and cladding silica, a silicon waveguide based on a silicon on insulator (SOI) material has an extremely strong binding effect on transmission of a light field, and the silicon waveguide has a very small section size under a single-mode condition, thereby implementing a miniaturized photonic device with high integration density. In addition, an SOI manufacturing process has advantages of being compatible with a traditional complementary metal-oxide-semiconductor (CMOS) process and being able to implement integration between a photonic device and an electronic device. Therefore, the SOI material has become a main material for preparing an integrated optical waveguide device and has a wide application prospect.
For an integrated optical waveguide device, a critical issue is that when an optical signal is input and output to implement communication between the device and an off-chip optical system, especially when current luminous efficiency of silicon does not meet a practical requirement, an optical fiber needs to be used to introduce a light source from the outside of an optical waveguide. However, the integrated optical waveguide has a strong binding capability for a light field, and during single-mode transmission, a spot size of the integrated optical waveguide is approximately on an order of hundreds of nanometers and a mode field shape of the integrated optical waveguide is generally an ellipse, while a common single-mode fiber has a weaker binding capability for the light field, and during single-mode transmission, a spot size of the common single-mode fiber is generally about 10 micrometer (μm) and a mode field shape of the common single-mode fiber is a circle. In this case, when the integrated optical waveguide is directly coupled to the single-mode fiber, there is a huge difference between the spot sizes of the integrated optical waveguide and the single-mode fiber, and the mode field shapes of the integrated optical waveguide and the single-mode fiber are seriously mismatched. In addition, a difference between refractive indexes of interfaces of the integrated optical waveguide and the single-mode fiber during coupling also causes an extra Fresnel reflection loss. As a result, when the integrated optical waveguide is directly coupled to the single-mode fiber, efficiency is very low and generally does not exceed 10%, which cannot meet a commercial requirement obviously. Therefore, development of a simple and effective method for input-output coupling between an integrated optical waveguide and a single-mode fiber that has high coupling efficiency, a low device requirement, and low package costs has great practical significance to practicability of an integrated optical waveguide device.
An inverted taper-based spot-size converter method is a commonly-used transverse coupling method. As shown in FIG. 1, a principle of the coupling method 100 includes using an inverted taper structure 121 whose width is gradually decreased and that is disposed at an end of a silicon waveguide 120 to enlarge a small-size spot of the silicon waveguide 120, and disposing, at the inverted taper structure 121 of the silicon waveguide 120, a low refractive index optical waveguide 130 wrapping the inverted taper structure 121 such that a mode field in the silicon waveguide 120 gradually separates from the silicon waveguide 120 and is transferred into the low refractive index optical waveguide 130, thereby enlarging a size of a mode field that transmits light to enable the size to be close to a size of a mode field of a single-mode fiber, and then forming a locating slot (for example, V-shaped slot) 150 at one side, opposite to the silicon waveguide 120, on an upper surface of a substrate 110, and fastening a cylindrical single-mode fiber 140 in the locating slot 150 such that a surface of one end of the single-mode fiber 140 is in contact with a surface of one end of the low refractive index optical waveguide 130, and a center line of the single-mode fiber 140 is perfectly aligned with a center line of the low refractive index optical waveguide 130.
The inverted taper-based spot-size converter method has relatively high coupling efficiency and a broadband coupling feature, and the coupling efficiency is insensitive to polarization that is for transmitting light. However, the coupling method has the following disadvantages. First, because a mode field diameter of the silicon waveguide that is enlarged using the inverted taper structure is still relatively small (about 3 μm), the single-mode fiber still needs to be perfectly aligned with the low refractive index optical waveguide. For example, if the single-mode fiber uses a lensed fiber whose terminal diameter is 3 μm, 1 decibel (dB) alignment tolerance between the single-mode fiber and the low refractive index optical waveguide is about ±0.3 μm. Therefore, it can be seen that the coupling method has a very high precision requirement on a packaging device. Second, due to the high alignment precision requirement of the coupling method, the coupling method usually requires active coupling to ensure coupling efficiency, which increases coupling complexity. Third, to improve the coupling efficiency, polishing processing generally needs to be performed on an end face of the low refractive index optical waveguide, and an anti-reflection film needs to be coated on the end face of the low refractive index optical waveguide, or refractive index matching liquids are added between the end face of the low refractive index optical waveguide and a terminal of the single-mode fiber to reduce Fresnel reflection on the end face. Fourth, because the coupling method requires very high alignment precision, the method is relatively difficult to implement coupling between an optical waveguide and a fiber array and is not applicable to coupling and packaging for an optical chip having high-density input and output interfaces. In summary, it is a problem that urgently needs to be resolved in the art as how to reduce a requirement on alignment precision on the basis of ensuring coupling efficiency between an optical waveguide and a single-mode fiber in order to lower a requirement of coupling on a device, reduce packaging costs, and implement coupling between an optical waveguide array and a single-mode fiber array.